Ameliorative Mechanisms of ZnO Nanoparticles in Salinity-Stressed Basil Callus: From Redox Defense to Metabolomic Normalization

Salinity is a major abiotic stressor inducing osmotic imbalance, ion toxicity, and oxidative damage in plant cells. This study investigates the ameliorative effects of 100 mg L−1 zinc oxide nanoparticles (ZnO NPs) on Ocimum basilicum L. callus cultures under salinity stress (0–90 mM NaCl) through an integrated enzymatic and GC–MS metabolomic approach. Exposure to 90 mM NaCl significantly suppressed biomass and increased oxidative stress markers (H2O2 and MDA). Notably, ZnO NP supplementation exhibited a selective modulatory role, preferentially stimulating the ascorbate–glutathione cycle, with ascorbate peroxidase activity reaching a peak of 1.16 U mg−1 protein. While ZnO NPs alone caused a slight increase in oxidative stress indicators, they significantly mitigated severe stress by reducing H2O2 levels and stabilizing protein content. Metabolomic profiling revealed that salinity triggered a shift toward sesquiterpene hydrocarbons (79.71%), whereas ZnO NP application reprogrammed secondary metabolism toward ester compounds (40.50%) and oxygenated metabolites, effectively normalizing salt-perturbed metabolite pools. Principal component analysis explained 72.5% of total variation, clearly distinguishing nanoparticle-driven recovery from stress-driven damage. These findings demonstrate that ZnO NPs enhance stress tolerance by reinforcing metabolic bridges between antioxidant defense and biomass preservation, offering a robust strategy for nano-enabled resilience.